Changes in behavior:
* The -P option is now based on PCRE2.
* The egrep and fgrep commands now warn that they are obsolescent and
should be replaced by grep -E and grep -F.
* The confusing GREP_COLOR environment variable is now obsolescent.
* Regular expressions with stray backslashes now cause warnings.
* Regular expressions or subexpressions that start with a repetition
operator now also cause warnings.
* Regular expressions like [:space:] are now errors even if
POSIXLY_CORRECT is set.
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